1. Field of the Invention
This invention relates to a process and apparatus for oxygen-enriched combustion of a fuel in an industrial furnace, in particular, a natural gas fired, regenerative glass furnace, which significantly reduces NO.sub.x emissions while improving production rate, glass quality, and furnace thermal efficiency, with no adverse effect on other air pollutant emission levels.
2. Description of the Prior Art
It is generally known that oxygen-enriched combustion, usually implemented by increasing the oxygen concentration of the combustion air or by direct injection of oxygen into the flame, increases production rate as well as furnace thermal efficiency. However, to provide such increases in production rate and furnace thermal efficiency, oxygen-enriched combustion increases the peak flame temperature, a condition which directly conflicts with known methods for reducing the formation of NO.sub.x. In addition, oxygen-enriched combustion is known to raise furnace refractory temperatures, particularly in the furnace zones where temperatures are already very high, thereby adversely affecting the service life of the furnace.
It is generally known that to control the amount of NO.sub.x emissions generated by a combustion process, it is necessary to reduce the amount of oxygen available for NO.sub.x formation. It is also generally known that peak flame temperatures in the combustion process must be maintained below the level required for substantial formation of NO.sub.x. Many approaches to producing such conditions to reduce NO.sub.x emissions generated in combustion processes, including oxygen-enriched combustion and staged combustion, have been used, investigated, or proposed.
U.S. Pat. No. 4,761,132 and related U.S. Pat. No. 4,909,727 relate to a process and apparatus for oxygen-enriched combustion which provides increased heat transfer to the furnace load, enhanced furnace specific production rate, improved furnace thermal efficiency and reduced emissions of NO.sub.x. The '132 patent teaches a process and apparatus for oxygen-rich combustion in which a first portion of fuel and an oxygen-rich gas are introduced into a cracking chamber and cracked to produce a cracked products mixture. The cracked products mixture, a second portion of fuel, and an oxidizer having sufficient oxygen for substantially completing combustion of the combustible portion of the cracked products mixture and the fresh fuel are subsequently introduced into a combustion chamber for completion of the combustion process. The '727 patent teaches a process for combustion as taught by the '132 patent where the process is carried out in a continuous regenerative furnace.
One approach to staged combustion involves air staging, a two stage combustion process, in which oxygen available for NO.sub.x formation is reduced by operating the initial stage of the combustion process at close-to-stoichiometric conditions, forming a close-to-stoichiometric primary combustion zone, removing heat from the primary combustion zone, and adding combustion air downstream of the primary combustion zone in the second stage of combustion to complete combustion. In addition to ensuring the absence of a sufficient amount of oxygen for high amounts of NO.sub.x formation, temperatures in the primary combustion zone are generally maintained below the level required for substantial NO.sub.x formation.
Reduction of NO.sub.x emissions in a regenerative glass melting furnace using air staging is taught by Abbasi, H. A. et al., Development of NO.sub.x Control Methods for Glass Melting Furnaces, Report for Gas Research Institute, Chicago, Ill., September 1983 in which preheated primary combustion air flow was reduced producing relatively fuel-rich combustion and ambient secondary combustion air was injected into the furnace proximate the furnace exit to burn out combustibles in the combustion products. Reduction of NO.sub.x emissions in a regenerative glass melting furnace using air staging is also taught by Barklage-Hilgefort, H., Reduction of the NO.sub.x -Emission of Glass Melting Furnaces by Primary Measures, Huttentechnische Vereinigung der Deutschen Glasindustrie (HVG), Frankfurt/Main, Germany, where a portion of the combustion air required for complete combustion is removed from the top of a regenerator and introduced into the furnace downstream of the flame. See also European Patent 0 306 657.
Staged combustion using air staging is also taught by U.S. Pat. No. 4,358,268 in which a preheated fuel-rich combustion mixture is fed into an industrial furnace system having a heating chamber, the mixture having been preheated in a first regenerator. Flue gases from the heating chamber are exhausted through a second regenerator into which is fed secondary combustion air to more completely oxidize the flue gases. A portion of the hot flue gases leaving the second regenerator is recirculated to the heating chamber and the remaining flue gases, having been fully oxidized, are vented to the atmosphere. The flow of flue gases within the heating chamber is periodically reversed such that the fuel-rich combustion mixture is heated in the second regenerator and the flue gases from the heating chamber are exhausted through the first regenerator to which secondary combustion air is added. Such reversal of flows is typical of the operation of regenerative-type industrial furnaces, including regenerative glass-melting furnaces. As taught by this patent, secondary combustion air is preheated in the regenerator through which the combustion products are exhausted and then reintroduced into the bottom of the regenerator to mix with the exhausting flue gases and complete combustion therein One disadvantage of this approach is the significant amount of additional ducting and blowers required for implementation. A second disadvantage arises from the fact that combustion is occurring within the exhaust regenerator, which combustion can produce localized hot spots within the regenerators and lead to premature deterioration thereof.
U.S. Pat. No. 4,874,311 teaches a method and apparatus for staged combustion of a fuel in a regenerative furnace in which a first portion of fuel and an oxidizing gas are combusted in an auxiliary combustion chamber in the furnace to produce a luminous stream of hot, pyrolized combustion products which are directed to mix with hot combustion air from a regenerator to create a final flame in the furnace above the material to be heated or melted. A controlled amount of fuel is also directed through the combustion chamber where it mixes with the hot combustion products from the auxiliary combustion chamber and the hot combustion air to form the final flame pattern.
U.S. Pat. No. 4,496,316 teaches a method and apparatus for selectively controlling the combustion air flow in a furnace firing port of a furnace having a plurality of firing ports. To enhance the flow of combustion air within the furnace, a small quantity of pressurized gas, for example, air, is injected generally along the flow path of combustion air in the plenum of the firing port to induce additional flow into the port. Combustion air flow is decreased by injecting air countercurrent to the combustion air flow within the plenum to impede flow through the port into the combustion chamber.
U.S. Pat. No. 4,666,403 teaches a system for preheating combustion gas for a continuous fired furnace using high temperature waste gases exhausted from the furnace.
U.S. Pat. No. 3,207,493 teaches a regenerative furnace having two combustion air inlets and a separate outlet for combustion products, reversing valves for alternately connecting the cold ends of the regenerators with the cold air supply and the exhaust flue, and operative to connect the hot end of one regenerator alternately with the combustion air inlets and the hot end of the other regenerator with the combustion product outlet of the furnace chamber.
In air staging for regenerative glass melters, in particular, fuel is generally burned at the firing port(s) with a portion of hot combustion air, primary air, at fuel-rich conditions, and the remaining air, secondary air, is added so that it mixes with the fuel-rich flame after a substantial amount of heat has been removed. The fuel-rich combustion retards NO.sub.x formation due to both lower oxygen availability and lower flame temperatures. Also, because the secondary air mixes with the flame only after heat has been removed, the secondary peak flame temperatures are also lowered. However, splitting the combustion air in a regenerative glass tank as well as in other high-temperature furnaces is difficult because it requires major modifications, and can reduce furnace production rate, increase electricity consumption, and reduce furnace thermal efficiency In addition, proper mixing of the secondary air with the primary combustion gases requires higher secondary air pressures which are not desirable in a regenerative glass melter.